One problem that arises when it is required to form an electrical (ohmic) contact to a base layer comprised of wide-bandgap semiconductor material, such as the Group II-VI alloy Hg.sub.1-x Cd.sub.x Te, is in forming a high conductivity contact. For values of x that are greater than approximately 0.4 it becomes difficult to match the work function of the base layer to the work function of the contact metal system. One undesirable result of a mismatch in work functions may be the formation of a non-ohmic (rectifying) electrical contact to the base layer.
Some conventional contact systems include one or more layers of metalization that are deposited directly upon the wide-bandgap base layer. In that the surface of the base layer will typically be exposed to atmospheric and other contaminants during the metalization process, and also possibly during a subsequent high-temperature anneal, there exists a potential to generate surface oxides, defects and barriers at the base layer/metal interface. These defects and barriers adversely impact the quality of the base layer and the electrical contact by inhibiting the transfer of charge carriers across the interface.
A further problem is created in that the presence of these impurities also adversely impact the matching of the work function of the contact metal to the work function of the underlying semiconductor material of the base layer. As was indicated above, a mismatch in work functions may result in the undesirable formation of a non-ohmic (rectifying) electrical contact to the base layer.
An alternate approach to forming the contact would form a cap or contact layer upon the base layer, and then deposit the contact metalization upon the contact layer.
For the case where the base layer is fabricated by LPE, conventional practice (for a large melt volume, Hg-rich LPE process) removes the base layer from the LPE growth chamber and then further processes the base layer to form the contact layer by, for example, vapor phase epitaxy (VPE), chemical vapor deposition (CVD), or metal-organic chemical vapor deposition (MOCVD). However, removing the base layer from the growth chamber exposes the layer to atmospheric contaminants which may adversely affect the quality of the resulting electrical contact. Furthermore, the additional processing steps tend to increase the cost and reduce the yield of the semiconductor component that includes the base layer.
An alternate LPE process to the Hg-rich, single melt process described above employs a plurality of Te-rich growth solutions to fabricate double layers of HgCdTe. As an example of this technique, reference may be had to an article entitled "Growth of Hg.sub.1-x Cd.sub.x Te heterolayers by slider LPE using separate compensating atmosphere of mercury", Journal of Crystal Growth 113, pgs. 520-526 (1991) by J. S Chen et al. As described in this article, two Te-rich HgCdTe solutions are required within the growth chamber. The solution that is used to grow the second layer is doped with indium to provide an n-p Junction between the two layers.